As semiconductor device features continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers at various stages throughout a semiconductor fabrication process. For example, dopant metrology processes are used at various steps during a semiconductor manufacturing process to monitor and control one or more semiconductor layer processes. In the case of optical metrology, the reflected energy that results when an optical beam is directed at a sample can be analyzed using a range of different optical techniques. A number of systems have been developed for the nondestructive evaluation of semiconductor samples in recent years. One such product is a Modulated Optical Reflectance (MOR) based system. A standard MOR tool includes an intensity modulated pump laser beam which is focused on the surface of a wafer sample for the purposes of periodically exciting the sample.
In the case of semiconductor wafers, thermal and plasma waves are generated in the sample, which then emanate from the pump beam spot. Changes in the reflectivity of the surface are caused by the generated thermal and plasma waves. Thermal wave and plasma wave monitoring systems typically are based on the detection of changes in intensity of a probe beam reflected off the surface of a semiconductor or other appropriate sample. Features and regions below the sample surface that alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be obtained.
A basic MOR system typically includes a second laser for generating a probe beam of radiation. This probe beam is focused collinearly with the pump beam and reflects off the sample. A photodetector is provided for monitoring the power of the reflected probe beam. The photodetector generates an output signal that is proportional to the reflected power of the probe beam and is therefore indicative of the varying optical reflectivity of the sample surface.
The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation frequency. For this purpose, the basic MOR system usually includes a lock-in amplifier which is also used to monitor the magnitude and phase of the periodic reflectivity signal. This output signal is conventionally referred to as the modulated optical reflectivity of the sample.
One stage of a typical semiconductor wafer processing cycle includes ion implantation. Ion implantation includes implanting dopant ions into surface regions of a semiconductor wafer. However, ion implantation tends to damage the crystal lattice of the semiconductor wafer. The damage is typically proportional to the concentration and depth of ions within the crystal lattice. This makes measurement of damage an effective substitute for direct measurement of dopant concentration and depth. MOR metrology systems have proven suitable for measuring damage and have been widely used for post implantation evaluation.
Another stage of a semiconductor wafer processing cycle includes annealing. In order to aid in reversing the damage caused by ion implantation a subsequent wafer annealing process step may be implemented. For example, a rapid thermal annealing (RTA) process may be performed using a laser system to heat a selected region of the wafer to a selected annealing temperature for a short amount of time (on the order of milliseconds).
Typically, MOR tools are used for MOR measurements on ion implanted wafers and annealed wafers originating from different sections of a fabrication facility. There is no information exchange between different MOR metrology tools monitoring different process steps of the same wafer. For example, there is no information exchange between a MOR tool measuring a semiconductor wafer prior to implantation and a MOR tool measuring the same semiconductor wafer after implantation. Further, there is no information exchange between a MOR tool measuring the semiconductor wafer after implantation and a MOR tool measuring the semiconductor wafer after an annealing process.
As a result, current semiconductor processing cycles do not provide for MOR comparison steps, wherein MOR measurements taken before or after one stage of the semiconductor process cycle are compared to another measurement taken from the same wafer at another stage in the process cycle.
Accordingly, it may be desirable to provide a dopant metrology system (e.g., MOR tool based metrology system), which provides information “feedforward” from a metrology tool measuring a wafer at one stage in a semiconductor processing cycle to another metrology tool measuring a wafer at a different stage in the process cycle. Additionally, it may be desirable to provide a dopant metrology system capable of providing information “feedback” to a process tool in order to adjust one or more process parameters of that process tool.